A drive force control system for a vehicle in which a continuously variable transmission is connected with an output shaft of an internal combustion engine is known in the art. The continuously variable transmission is capable of controlling a speed change ratio between an input speed and an output speed steplessly so that a driving condition of the internal combustion engine can be controlled to improve a fuel economy. An example of the control system for a vehicle in which the continuously variable transmission is connected with the output shaft of the internal combustion engine in a power transmittable manner is disclosed in Japanese Patent Laid-Open No. 2006-125602. According to the teachings of Japanese Patent Laid-Open No. 2006-125602, a belt-type continuously variable transmission is connected with an output shaft of the engine through a torque reversing mechanism provided with a forward clutch functioning as a starting clutch and a reversing brake.
The aforementioned belt-type continuously variable transmission comprises a primary pulley, a secondary pulley, and a belt applied to those pulleys. The primary pulley is provided with a primary pulley, therefore, a speed change ratio of the belt type continuously variable transmission can be controlled continuously by controlling an oil pressure applied to the primary pulley using a hydraulic control unit. Meanwhile, the secondary pulley is connected with a drive wheel through a differential gear and a drive shaft. In addition, according to the teachings of Japanese Patent Laid-Open No. 2006-125602, the vehicle further comprises: an engine controller for controlling the engine electronically; a transmission controller for electronically controlling the hydraulic control unit connected with the torque reversing mechanism and the belt type continuously variable transmission; and sensors connected with the transmission controller.
In the event of starting the vehicle, a torque capacity coefficient is set on the basis of a driving condition of the vehicle, that is, on the basis of a speed ratio of the starting clutch and an opening degree of an accelerator. Therefore, a torque capacity of the starting clutch is controlled on the basis of the torque capacity coefficient and a rotational speed of the engine. Specifically, in case a large acceleration force is demanded by a driver, the torque capacity is corrected to be reduced. For this reason, in case the driver intends to accelerate the vehicle abruptly, the rotational speed of the engine can be raised promptly to a region where a large torque can be generated.
In addition, a control device for engine connected with a transmission is disclosed in Japanese Patent Laid-Open No. 2006-112248. The transmission taught by Japanese Patent Laid-Open No. 2006-112248 is provided with various kinds of clutches functioning as speed change elements, and the transmission is configured to carry out a speed change operation by engaging and disengaging those clutches. Under an automatic shift mode, an optimal gear of the transmission is determined on the basis of an accelerator operation amount and a vehicle speed with reference to a map. To the contrary, under the manual shift mode, the gear is manually shifted to a higher gear or lower gear stepwise in accordance with a downshifting operation and an upshifting operation of a driver through a shift lever. Specifically, in case of carrying out a downshifting of the transmission, a feedback control of an engine output is carried out while disengaging the clutch to neutralize the transmission thereby adjusting the engine speed to a speed to be attained after the downshifting, and then, the clutch is engaged after synchronizing the engine speed.
However, provided that the speed change ratio of the continuously variable transmission taught by Japanese Patent Laid-Open No. 2006-125602 is changed stepwise as under the manual shift mode taught by Japanese Patent Laid-Open No. 2006-112248, shift shocks may occur during the speed change operation of the continuously variable transmission.